The hero's side, unlike the villains, get paid for what they're doing and Dark Blood is one greedy fucker.
Dark Blood is a Glory Seeker who wants to be admired, loved, & respected.
Dark Blood can't get away with killing innocents, but he can get away with killing villains (the corollary of the 'Bitterleaf Argument') and Dark Blood decides to use this to his advantage & join the heroes.
Lampshaded: Captain Awesome remarks on how Dark Blood is more evil than many villains they fight.
Invoked: Dark Blood knows he'll have more opportunity for mayhem if he fights with the heroes, so he asks to join them.
Exploited: The villain, Mystery Murder, wants to convince one of the heroes to switch sides or tell him/her about the rest of the team's weaknesses. Aware of Dark Blood's immorality, (s)he chooses him, thinking that he will be the easiest to convince.
Captain Awesome and Amazing Girl don't let Dark Blood join them because that character is evil.
Once Dark Blood joined the team, he decided to undergo a Heel-Face Turn and become a better person.
"Why is it every team I've been in has a villain or a jerk (or Crystal Dragon Jesus Save Us, both)?"
"Everyone on this team is so nice! ... Wait! Does that mean I'm the Token Evil Teammate? Crap! I suck at being a Jerkass!"
Conversed: "Wait a minute, why is there a villain helping out the heroes?"
Dark Blood gets kicked off Team Good because Captain Awesome and Amazing Girl are fed up with his insatiable bloodlust.
A bunch of nerds play a tabletop RPG, and most of them play heroic characters. At the start of a new campaign, one of them rolls an evil character, causing uneasiness in the group. Ultimately, the player playing the evil character touches the trigger of another player, resulting in real life drama.